Deionized water was oxidized to form ozone at the anode while oxygen was reduced to hydrogen peroxide at the cathode in a proton-exchange-membrane electrochemical flow reactor. The conditions for simultaneous generation of these oxidants were determined as a function of the applied voltage, electrode materials (lead dioxide for ozone evolution; gold, carbon, or graphite for peroxide evolution), and electrode configurations. Measured and calculated quantities included cell current, liquid- and gas-phase ozone concentrations, hydrogen peroxide concentrations, current efficiency for ozone and peroxide evolution, and ozone and peroxide production rates. An applied potential of 4.5 V resulted in a current density of 2 A/cm2, yielding maximum gas- and liquid-phase ozone concentrations of 60 and 3.1 mg/liter at the anode (4.5% current efficiency) and hydrogen peroxide concentrations between 3 and 5 mg/liter at the cathode (0.8% current efficiency).